Stock exhaust manifolds vs headers

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tandart66

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Hey all,
I'm in the process of rebuilding the slant for my 66' dart, and am currently debating whether I should run headers or stay with the stock exhaust manifold. Currently the plans for the build include a 4bbl offy intake, 600 cfm Eddy carb, overbore pistons (machine shop has to assess damage to the bores then will give me a number), and a mild performance cam. My goals for the car is to be able to road trip, drive it on a somewhat regular basis, and be able to push it in the canyons on the weekends. Should I go with the stock manifold with a bigger pipe or go with some headers (Clifford most likely)?
 
tandart66 said:
Hey all,
I'm in the process of rebuilding the slant for my 66' dart, and am currently debating whether I should run headers or stay with the stock exhaust manifold. Currently the plans for the build include a 4bbl offy intake, 600 cfm Eddy carb, overbore pistons (machine shop has to assess damage to the bores then will give me a number), and a mild performance cam. My goals for the car is to be able to road trip, drive it on a somewhat regular basis, and be able to push it in the canyons on the weekends. Should I go with the stock manifold with a bigger pipe or go with some headers (Clifford most likely)?
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I would go with headers, for sure. You also might want to consider raising the compression of your engine as well!
 
My vote is to get a stock flange gasket and use it to compare to the size of the original style manifold and take a die grinder and port the manifold to match the size of the outlet gasket,
When I did mine I couldn't believe both the extent to which the cast hole in the manifold was 1)smaller than the outlet gasket and
2) off center relative to the studs.
I bolted the gasket on, and took a die grinder and "went to town". At one point the original cast hole touched the edge of the gasket and opposite I took out about 5/16-3/8" of cast iron to even out the exhaust hole.
And tapered down from that to nothing, where the flange gasket touched the existing port hole. I basically had to grind material around about 3/4 of the circumference of the exhaust flange to accomplish this.
The hole in the flange gasket I used is 2-1/4 diameter.
Which is a super common size of upgrade for a /6 exhaust system.
When you dump 1-7/8 worth of exhaust into a 2-1/4 pipe that is a pretty stout restriction/choke point. But opened up to match the head pipe will be a noticeable improvement.
RRR had a thread on doing just this as well
And there's plenty of meat left in the manifold when done
I know that/6 manifolds were very prone to cracking. I wonder if this bottleneck to flow may have contributed to that tendency back in the day...
 
Rick posted while I was hunting and pecking, I agree with the compression increase. All pros and no cons unless you get crazy.
Between the block and head I used, I had 0.100" milled off in my rebuild and ended up with exactly the 8.4:1 that these engines were advertised as having been, in the first place.
Meaning that as built, stone stock, the fell way short of "advertised".
I was actually going for closer to 9:1 but having bought and started with a basket case of a pile of /6 parts that became my engine I had no baseline measurements for depth of piston top at tdc, etc to go off of, so the amount milled off of my head and block was a wild assed guess as to how much to mill off.
 
Only you can really answer that question is the performance gain worth the cost among the other factors to you ?
 
273 said:
Only you can really answer that question is the performance gain worth the cost among the other factors to you ?
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How much will really be gained in this case?
Stock manifold vs ported manifold vs headers?
And at that 6 into 1, or 3-2-1, twin sets of 3, or whatever other variation of headers that could be had?
 
volaredon said:
How much will really be gained in this case?
Stock manifold vs ported manifold vs headers?
And at that 6 into 1, or 3-2-1, twin sets of 3, or whatever other variation of headers that could be had?
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Not much, headers are an arguable mod on a mild V8's, that's why said the OP the only one that can answer it what does he value more $$$$ or a header on his car with probably very little performance gain.
It really depends on the person for some the cost doesn't matter others it does.
 
273 said:
Not much, headers are an arguable mod on a mild V8's, that's why said the OP the only one that can answer it what does he value more $$$$ or a header on his car with probably very little performance gain.
It really depends on the person for some the cost doesn't matter others it does.
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The OP said he wanted a 4bbl intake & carb with a mild cam, whatever that means. What I've read the /6 would do well with a smaller Carter style carb than 600cfm. I would probably go with a 500CFM AVS2. Smaller is almost always better. The cam could be the key to what he wants out of the car.
 
Here's a basic cam 4bbl & header plus rebuild goes from 71 RWHP to 132 RWHP.





Here's more wild 282 hp

 
273 said:
Here's a basic cam 4bbl & header plus rebuild goes from 71 RWHP to 132 RWHP.





Here's more wild 282 hp

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The second video was a good one for the OP to watch. From 70 to 132HP after a rebuild with good aftermarket parts was a nice improvement.
 
Since the slant six is "two holes down" it's a much more systematic approach than a V8. INstead of putting on one part at a time and seeing results, the slant six needs all the systems upgraded to see good results. I had the stock manifold on mine ported and the outlet opened up to 2.5" and ran 2.5" pipe all the way back. It made a nice difference. Mine is also over 10:1 compression, has a 450 Quick Fuel four barrel and a big solid lifter cam.
 
volaredon said:
Rick posted while I was hunting and pecking, I agree with the compression increase. All pros and no cons unless you get crazy.
Between the block and head I used, I had 0.100" milled off in my rebuild and ended up with exactly the 8.4:1 that these engines were advertised as having been, in the first place.
Meaning that as built, stone stock, the fell way short of "advertised".
I was actually going for closer to 9:1 but having bought and started with a basket case of a pile of /6 parts that became my engine I had no baseline measurements for depth of piston top at tdc, etc to go off of, so the amount milled off of my head and block was a wild assed guess as to how much to mill off.
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What would I have to do to reach 9:1/9.5:1 CR? Only thing ive told the machine shop to do thus far is to overbore it to get all the damage cleaned up (probably 0.030).
 
tandart66 said:
What would I have to do to reach 9:1/9.5:1 CR? Only thing ive told the machine shop to do thus far is to overbore it to get all the damage cleaned up (probably 0.030).
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I believe RRR & Volaredon had their cylinder heads cut 0.100 to raise the comp ratio. RRR said his comp ratio is now over 10:1.
 
Depending upon your budget... I would prioritize head work, around the valves, or larger valves before spending $$ on headers... for two reasons.. One it allows for headers later if you want more performance, AND the head work is already done.. If you spend the money on headers, with small valves and no bowl work, then the headers won't add as much..

Most of the speed and performance parts for a slant six are limited by head flow... Make sure that the head gets the attention it needs, and then throw the intake and exhaust parts on it after a year or two if more speed is needed. Well the factory 2bbl intake made a difference!

Now I am in the process of building my own Dutra Duals, and converting a stock cast iron intake manifold to a spread bore design.. I have not done any head work.. BUT I mostly have labor invested, and a good running engine.. If I was doing a rebuild I would be focused upon head work as much as the short block.

A couple of fun articles on the slant six.. be very aware that there is a cheaper way ( and better i would argue) to do the ignition, intake manifold, and Dutra Duals. Machine work is going to be the same on all full rebuilds.. AND understand that the articles are largely an advertisement for the 'speed parts'.... still it's fun reading and shows what others have done with lots of pictures...

A three part series: https://www.motortrend.com/how-to/making-mopar-slant-six-sense-part-3/
And a bowl hog cutter was used under the stock valves here, as well as good head work: https://www.motortrend.com/how-to/add-77-horsepower-mopar-slant-six-basic-bolt-ons/
 
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RustyRatRod said:
Since the slant six is "two holes down" it's a much more systematic approach than a V8. INstead of putting on one part at a time and seeing results, the slant six needs all the systems upgraded to see good results. I had the stock manifold on mine ported and the outlet opened up to 2.5" and ran 2.5" pipe all the way back. It made a nice difference. Mine is also over 10:1 compression, has a 450 Quick Fuel four barrel and a big solid lifter cam.
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I’ve seen a video of your car running and it sounds great! What are the cam specs on it?
 
Topless69 said:
I’ve seen a video of your car running and it sounds great! What are the cam specs on it?
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Thanks for the kind words.

It's a custom regrind from Oregon cams. I needed to utilize some springs I already had and they were small single Comp 942. I had them new. They were left over from a project that I didn't complete years ago, so I used them. They needed low lift and that's fine with me. Less stress on everything. So the grind I settled on is .465 lift and 250 @ .050 duration ground on a 108 center line. I had to go big on the duration because of what happened to the cylinder head. It was "accidentally" milled like .180" and got compression way higher than I wanted so I knew to run on pump gas I needed something pretty stout duration wise. It turned out really good, much to my surprise. lol
 
Ricks70Duster340 said:
I believe RRR & Volaredon had their cylinder heads cut 0.100 to raise the comp ratio. RRR said his comp ratio is now over 10:1.
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Mine is 0.020 over (I wanted to bore the least amount as possible to eliminate the wear and taper that oversized parts were available for)
No sense to go "straight to 60 over" unless your block actually needs 60-over to correct for wear
I had my block decked 0.030 and the head 0.070.
I guessed on the block since the motor was in pieces missing rods and pistons when I got it.
A guess based on how far I had heard on the forums that a stock 225 "usually is" in the hole at tdc.
So are my pistons (stock style sealed power) shorter than the originals or was my block "tolerance stacked" and "taller than spec" from the factory? Or both?

I then found out that most "rebuilder grade" pistons are actually a shorter deck height than stock to "compensate" for any chance of a CR increase because of the bigger diameter however minimal that increase might be "thanks EPA". Most stock unrebuilt 225s are claim d to be about 0.180 in the hole at TDC.
Mine is that, after the 0.030 shave. So had I not shaved the block I would have been at 0.210 in the hole. I was hoping for 0.120-0.150 max but I wasn't pulling it back apart again and sending it back to the machine shop, once I had the short block home and assembled.

I then had the machine shop CC the head ( because they had it at that point, not me) and tell me what it came out to, and decided on a 0.070 shave on the head to get the chamber CC volume where I wanted it to tailor the compression ratio.
I cc'd the head when I got it back and the head ended up where I had hoped it would actually.
I also added SI brand oversized valves at 1.70/1.44, and ported the head the best I could, as far as I dared without chancing breaking thru into the water jackets as then the head would be scrap metal with lots of time and grinding dust "for nothing'.


There's a series of articles on a 67 dart with exceptionally low miles that was donated to a community college in PA and these articles tell of the headaches they went thru to get it going. It had sat for years and the original engine was seized, they detailed the build and reasons for what was done. That engine ended up with pistons at .210 in the hole. They discovered it after getting it running and were disappointed in Dyno numbers compared to "what was advertised bach in the day". I think at one point they pulled the head and shaved it to increase compression for more power.
One difference between my engine and RRRs is mine is in a d150 and his is in a light assed 64ish valiant. Our end goals were the same/ as in improving performance, but yet different in other ways because of our respective applications.
Some mods are universal no matter your goal while others differ because of them.
 
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A little off track, excellent videos. Especially like the Weber carbs with the interchangeable Venturi’s. Back in the day, zenith carburetors were the same, highly tunable.

As for your build, those Aussie speed cast headers look like a viable alternative to the stock crack prone, poor flowing stock exhaust manifolds.
 
IDK what you should do;

But
with your other go-fast goodies, and
pistons up a lil higher than the deck, and
if your cam has more that 30* of overlap, I know that
I (me) would install long-tube Headers, like the Tri-Ys, into a long Transition, then
a long torque-tube, into one long, glass-pack-style muffler with NO reducers; then a whatever-fits tailpipe more than ~2"dia, with possibly, another glass-pack resonator on the cold-end beside the gas-tank, ending, finally, in a rectangular 340-style tip.

If I was on a budget,
I would do this before I bought the 4bbl kit.
But then.
if I was really on a budget,
You guessed it,
I would just install any old SBM/A904/2800stall, for way less money spent, and way more horsepower to burn.
 
Where do you want the power and what cam are you using? High compression and wild cam will benefit WAY more with headers than a 9:1 with a cam slightly larger than factory.
 
Jim they played your Dart Video on Motor Trend (My Classic Car) today. Amazing Car
 
TF360 said:
Jim they played your Dart Video on Motor Trend (My Classic Car) today. Amazing Car
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Thanks. It’s been on their YouTube channel for several weeks now. They did an individual car one and then a few weeks ago they put them in a single show. I like that format better. I wasn’t sure if was still on MotorTrend.
 
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